Brown Fat: An Energy Dissipating Tissue

This chapter is not intended to be a review on brown adipose tissue, but rather the problem of how and under what conditions this potential can be called upon by the organism.

Abstract

This chapter is not intended to be a review on brown adipose tissue. Throughout the different chapters of this book, the mitochondriologist, the nutritionist, the endocrinologist and the pharmacologist have described facets of this tissue and this adequately covers the metabolic potential of brown fat. In this chapter, I will be concerned with the problem of how and under what conditions this potential can be called upon by the organism. Brown fat mitochondria are able to develop tremendous respiratory rates, and this requires equally high substrate supply, oxygen delivery and heat transfer capabilites. These flows must be matched in some way,both in the steady state and dynamically, otherwise an acute.shortage of either substrate or oxygen could develop. A shortage of substrate at high respiratory rates leads to a failure of cellular ATP production and cellular ionic regulation. A shortage of oxygen supply on the other hand, besides limiting the heat production, would also lead to a collapse of cellular ATP synthesis, unless the brown adipocyte exhibited a Pasteur effect, which is not the case — at least in tissue from adult animals. The dynamic matching of supply to utilization therefore requires a concerted regulation of blood supply and cellular activation, which is probably mainly neural. When tring to lntegrate the biochemical, physiological and behavioural information relating to brown fat, one soon realizes that this is available mostly for one species, namely, the laboratory rat. Thus, one has to focus on the rat as a frame of reference.